Interference, or cross talk, occurs between two closely spaced signal lines. The cross talk is caused by electromagnetic fields generated by the signal lines. Due to cross talk, noise is generated in the signal lines and affects the functioning of the circuit.
The problem will become worse in future generations. Due to the scaling down of device geometries in deep-sub-micron technologies, interconnect wires are placed in increasingly closer proximity and higher density. With increased use of stacked dies and system in package (SIP) technologies, cross talk between chips increases significantly due to the small distance between chips. As a result of the increasing circuit density, coupling noise has increased significantly, and has become an important concern in high performance circuit design. If not compensated for, coupling noise may cause signal delays, logic errors, and even circuit malfunctions.
Cross talk can be reduced using isolation structures, particularly grounded metal lines or metal planes. FIG. 1 illustrates a conventional coplanar structure for reducing cross talk. Signal lines 4, 6 and signal-grounded lines 2 are located on the same plane. The respective electromagnetic fields generated by the signal lines 4 and 6 are partially insulated by signal-grounded lines 2 and thus the cross talk is reduced.
Although the structure shown in FIG. 1 is simple from a design and manufacturability perspective, the cross talk isolation effect is not satisfactory. For better isolation effects, a cable-like structure may be adopted. FIG. 2 is a cross sectional view of the structure. Three metal layers are separated by dielectrics 18. A signal line 8 is formed in the middle layer between two signal-grounded lines 14 and 20. In addition, two signal-grounded metal planes 10 and 12 are in metal layers above and below the signal line 8, respectively. Signal-grounded metal lines 14, 20, and metal planes 10 and 12 form an enclosed structure similar to a cable. This structure has a good cross talk insulation effect.
Conventional solutions suffer some drawbacks. In integrated circuits, space consumption is one of the major concerns. The conventional solutions have significant space consumption; therefore, their usages are limited. Additionally, signal lines are typically wired from devices in the substrate to a topmost metal layer through multiple layers. Adding grounded structures thus increases the complexity of the circuit design (e.g., may require additional metal layers).
What is needed, therefore, is a structure that provides sufficient cross talk isolation without the negative consequences of prior art solutions.